Radio transmitting circuit



April 1929- E. L. NELSON ET AL 1,710,235

RADIO TRANSMITTING CIRCUIT Filed Oct. 15, 1924 0A PA lnvenfars: fdn ard L A's/s00 Arf/Iur A. draw/d Patented Apr. 23, 1929.

UNITED STATES lflhhfilli PATENT OFFICE.

EDWARD L. NELSON AN D ARTHUR A. OSWALD, OF EAST ORANGE, NEVV JERSEY, AE-

SIGNORS TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. 531,

A CORPORATION OF NEW YORK.

RADIO TRANSMITTING CIRCUIT.

Application filed October 13, 1924.

This invention relates to high frequency wave transmission and particularly to radio transmission.

An object of the invention is to improve the wave form of the transmitted wave in systems of the type indicated.

More specifically, an object of the invention is to suppress the transfer of harmonics to an antenna circuit.

In accordance with the invention thereis interposed between the wave generating circuit and the antenna of a radio transmitting circuit a coupling circuit selective of the waves to be radiated. This coupling circuit is of such character as to efficiently transfer waves of the desired radio frequencies to the antenna, while at the same time it effectively prevents undesired harmonic frequencies from being impressed on the antenna.

In the single figure of the drawing, a typical. radio transmitting circuit embodying the invention is illustrated in schematic form.

Referring to the drawing, a voice input circuit, comprising the telephone transmitter 1 and the voice frequency amplifier VA is shown connected to the input circuit of a vacuum tube amplifier OA, acting as a modulator, which is supplied with high frequency carrier waves from a source 2. The output circuit of the amplifier OA is connected to a power tube amplifying system. PA, the output circuit of which is connected to the tuned circuit T C, which is in turn. coupled to an antenna circuit A.

The source 2 and its circuit for supplying the high frequency carrier waves to the amplifier OA may be of any suitable type, but is preferably such as that disclosed in Patout No. 1,356,763, dated October 26, 1920, issued to R. V. L. Hartley. The modulating systcn'l. comprising the transmitter 1, the Voice frequency amplifier VA and the am 'plifier OA is in accordance with that of Patents Nos. 1 142,146 and 1,442,147, dated January 16, 1923, issued to R. A. Heising'; the modulating system and power amplifying systen'i PA in accordai'lce with that of Patent No. 1,388,450 dated August 23, 1921, issued to E. H. Colpitts and H. D. Arnold. If the invention is applied to a high power system, the power amplifier circuit PA should preferably comprise a group of watencooled anode power tubes.

Home current is supplied train a Serial No. 743,231.

8 to the voice frequency amplifier VA and the amplifier OA in common through the choke coil 9. The amplified speech waves from the amplifier VA act in the manner described in the Heising patent, referred to, to modulate the waves supplied from the source 2 in the amplifier circuit 0A, and the resulting modulated waves are impressed on the grids of the power tube amplifier PA.

The tuned circuit TO, interposed between the power amplifier circuit PA and the antenna circuit A, comprises an adjustable inductance 3, a variable condenser i and a. variable condenser 5. The circuit is tuned to the frequency of the waves to be radiated. The antenna circuit A comprises an aerial or radi acting element 7 connected to ground through the tuning coil 6 and the variable condenser 41;. The variable condenser i is, therefore,

common to both the tuned circuit TC, which is the output circuit of the power amplifier PA, and to the antenna circuit A, and serves as a coupling element for transferring, energy from the power amplifier output circuit to the antenna. The degree of coupling is, of course, dependent 011 the size of the coupling capacity and, since ordinarily loose coupling is desired, the capacity of the coupling, condenser 4; is made relatively large compared to the other capacity in the tuned circuit TC.

Because of the distorting, effect present in generating and transmitting systems of the character illustrated, there will be produced in the output circuit of the power amplifier PA undesired oscillatory impulses harmoni cally related to the frequency of the carrier waves from the source 2. As the tuned circuit TC and the antenna circuit A are both tuned to the fundamental frequency, waves of this frequency will be readily transferred to the antenna and their amplitiilde will be large. Now, in order to obtain in the antenna circuit a wave of maximum purity, it is necessary to reduce the transfer of waves of the undesi red harmoi'iic fre niencies from the output circuit of the amplifier PA to the antenna circuit A. This result is obtained in accord ance with the invention by a suitable arrangement of the reactances in the coupling and antenna circuits as will now be described.

It will be observed that the tuned circuit TC as viewed from the amplifier PA has two ches. One branch compris s i ti, at ,1

a by-pass to high frequency components. The other branch includes the inductance 3 and condenser 4, which is relatively large, in series, For the higher frequencies, therefore, the path 3, t offers a relatively high impedance while path 5 offers a relatively low impedance with the result that the harmonies are largely diverted through the path 5 and their effect upon the antenna is correspondingly reduced. The use of a mutual capacity, the reactancc voltage across which serves as the. antenna driving voltage further tends to discriminate against the transfer of harmonic components to the antenna circuit since the driving or reactance voltage being inversely proportional to the frequency becomes relatively smaller and smaller for these higher frequencies.

Although the invention has been illus-' trated and describedin connection with the use of asingle condenser as the coupling ele ment in' theinductive' branch of the tuned circuit, it is to be understood that the invention is not necessarily limited to a single condenser but that a more complex circuit having a combination of impedances including inductances and capacities, such as a filter des1gned to discrnnlnate against particular frequencies or hands of frequencies, may

be employed as the coupling member.

The coupling circuit of this invention has the added advantage that when there is used anoutput circuit (that is, antenna circuit) having an impedance which is substantially constant with frequency, the tuning may be varied without changing the input impedance of the coupling circuit by varying only the inductance of the element 3 and maintaining the capacities of the condensers l and5 constant. I I

It is, of course,'understood that although the invention has been illustrated and described in connection with its use in radio transmitting circuits, the invention is not prising a condenser across said source of waves, a second condenser of relatively large capacity compared with said first condenser connected. across the load circult, and an in ductance forming in series with said second condenser a shunt across sa1d first condenser. 2. In a radio transmltting clrcuit, means :for generating Waves of radio frequencies,

a closed oscillatory circuit supplied with said waves, sald-clrcuit being separate from said means and including a condenser, an antenna, 7

pacity and having a high impedance to harmonic frequencies of sald waves, the other of said branches including saidcondenser and having a con'lparatively low impedance to harmonic frequencies of said waves.

3. In combination, an antenna circuit, a

source of waves, a closed oscillatory circuit separate from said source but tuned to the frequency of the oscillations to be radiated by said antenna circuit, said oscillatory circuit coupling said source to said-antenna and having two branches connected in parallel,

one of said branches comprising an inductance and a capacity in series, said capacity being common to said oscillatory circuitand said antenna vcircuit, the other of sald branches also including a capacity which is small compared to said first mentioned capaclty, whereby said first mentloned branch presents high impedance to oscillations of frequencies higher than those to be radiated andcooperates with said second mentioned branch to substantially prevent oscillations of said higher frequencies from being transferred to, saidantenna circuit.

4. In combination a source of waves, an

antenna circuit, and separate from said source circuit tuned to the frequency of the. waves to be transmitted for. coupllngsaid source to said antenna, said coupling circult comprising a' condenser connectedacross said source of waves, a second'conde'nser connected across the antenna, and an inductance forming in series with said second condenser a shunt across said first condenser, said second condenser having a capacity which is large compared'to the capacity of said first condense 1 i In witness whereof, we hereunto subscribe our names this 24th day of September, A. D

1924, and this 10thda-y of October, 1924:.

EDWARD L, NELSON. ARTHUR A. OSVVALD'. 

